1. Field of the Invention
The present invention relates to a ceramic capacitor, and more specifically it relates to a ceramic capacitor employed as a smoothing capacitor for a switching source.
2. Discussion of Background
Up to the present time, most smoothing capacitors for switching sources have been constituted of aluminum electrolytic capacitors. However, as the demand for both miniaturization and improved reliability have increased in the market, the need for a compact ceramic capacitor assuring a high degree of reliability has also increased.
Generally speaking, since a great deal of heat is generated in the vicinity of a source, substrates are normally constituted of an aluminum having a high heat discharge capacity. However, since the temperature in the vicinity of the source changes greatly when the source is turned on and off, a great deal of thermal stress occurs at a ceramic capacitor mounted on the aluminum substrate, which has a high coefficient of thermal expansion. This thermal stress causes cracking to occur at the ceramic capacitor, which, in turn, may induce problems such as shorting defects and arcing.
In order to prevent problems such as arcing, it is crucial that the thermal stress occurring at the ceramic capacitor be reduced. As a means for reducing the thermal stress, Japanese Examined Utility Model Publication No. 46258/1993, Japanese Unexamined Patent Publication No. 171911/1992, Japanese Unexamined Patent Publication No. 259205/1992 and the like disclose a structure achieved by soldering a metal plate onto a terminal electrode of the ceramic capacitor and mounting the metal plate onto the aluminum substrate to prevent the ceramic capacitor from being soldered directly onto the aluminum substrate.
Under normal circumstances, it is necessary to set the length of the leg portion of the metal plate extending from the terminal portion to be soldered onto the aluminum substrate to the portion where it is connected to the ceramic capacitor as large as possible in order to ensure that the thermal stress caused by the expansion and contraction of the aluminum substrate is absorbed to a sufficient degree. However, since products in the prior art adopt a structure in which the height of the ceramic capacitor is bound to increase if the legs of the metal plate are lengthened, the length of the leg of the metal plate must be restricted to ensure that it is less than the allowable height that is permitted on the substrate.
Because of this, the length of the legs of the metal plate cannot be set at a large value in the products in the prior art and, consequently, if the ceramic capacitor is continuously operated over an extended period of time in an environment where the temperature changes drastically (-55.degree. C. to 120.degree. C.), as in the vicinity of a source, cracks will occur near the ends of the ceramic capacitor, presenting a high risk of arcing. This gravely compromises the reliability of the ceramic capacitor and has been a obstacle to the wider use of ceramic capacitors.
In addition, the metal plate in the prior art is constituted of phosphor bronze, silver, copper, stainless steel, aluminum, nickel silver or the like. However, these metals all have a coefficient of average linear expansion that is markedly higher than the coefficient of average linear expansion of the ceramic dielectric material constituting the ceramic capacitor. Thus, if any of them is employed to constitute a component to be mounted in the vicinity of a source where the temperature changes greatly, a great deal of stress is applied to, in particular, the area where the metal plate is connected due to the difference between the coefficient of average linear expansion of the ceramic capacitor element and the coefficient of average linear expansion of the metal plate to result in cracking occurring near the ends of the ceramic capacitor, which may lead to problems such as continuity defects, arcing and the like.
Furthermore, ceramic capacitors achieving a large capacity by laminating a plurality of laminated ceramic capacitor elements, soldering metal plate terminals onto terminal electrodes of the individual laminated ceramic capacitor elements and electrically connecting in parallel the plurality of laminated ceramic capacitor elements have been proposed (e.g., Japanese Unexamined Patent Publication No. 188810/1992, Japanese Unexamined Patent Publication No. 17679/1996).
Normally, soldering paste containing solder particulates, rosin-type resin, an actuator and the like is employed to solder and secure metal plate terminals onto the terminal electrodes of laminated ceramic capacitor elements. The activator is constituted of a halogen compound containing chlorine and the like. The particle size of the solder particulate is set at approximately 1 .mu.m to 50 .mu.m. The rosin-type resin content is set within the range of 50 wt % to 55 wt %. The content of the activator which is constituted of a halogen compound containing chlorine and the like is set at approximately 1%. In addition, the distance formed between the individual capacitor elements when combining the laminated ceramic capacitor elements is maintained within a range of 10 .mu.m to 20 .mu.m.
However, when soldering the metal plate terminals onto the individual terminal electrodes of the laminated ceramic capacitor elements, the solder particles and the flux contained in the soldering paste enter the gaps between the laminated ceramic capacitor elements to result in buildup occurring due to the solder balls and the flux, presenting problems such as shorting defects between the terminals and deteriorated insulation.